For industrial production of a vinyl-based polymer, such as a vinyl chloride resin, suspension polymerization is widely applied in which a vinyl-based monomer, such as vinyl chloride, is dispersed in an aqueous medium using a dispersion stabilizer to be polymerized in the presence of an oil-soluble catalyst. In general, factors to control the quality of a vinyl-based polymer include a rate of polymerization, a polymerization temperature, a type and an amount of a catalyst, a model of a polymerization vessel, a stirring rate, a type of a dispersing agent, and the like. Among all, the type of a dispersing agent has a significant influence. For suspension polymerization of vinyl chloride, a vinyl alcohol-based polymer (a vinyl alcohol-based polymer may be abbreviated below as a PVA) is mainly used as a dispersion stabilizer. In this case, a partially saponified PVA is preferably used as a PVA. Suspension polymerization of vinyl chloride in an aqueous medium using a PVA as a dispersion stabilizer produces a vinyl chloride resin in the form of particles.
Such a dispersion stabilizer is required to have the following performances: (1) that the number of coarse particles in a polymer produced therefrom is small; (2) that the polymer produced therefrom has a sharp particle size distribution; (3) that the polymer produced therefrom has a high bulk density and a high mass per unit volume; and the like.
Since it is difficult to satisfy the above performances with a general partially saponified PVA, various attempts have been made. For example, Patent Document 1 discloses a dispersion stabilizer made by introducing acetaldehyde into an end of a vinyl alcohol-based polymer and applying heat treatment. Although the performances of the dispersion stabilizer are improved by the heat treatment compared with conventional products, they do not fully satisfy the above required performances.